The present invention relates to a method and apparatus for testing a thin-film magnetic head having at least magnetoresistive effect (MR) read head element.
Recently, it has been requested to improve performance of a thin-film magnetic head so as to satisfy increased recording density in a hard disk drive (HDD) unit. Therefore, a composite thin-film magnetic head with an inductive write head element and a MR read head element layered on the inductive write head element has been broadly utilized instead of an inductive read/write head.
Now, a thin-film magnetic head with a giant magnetoresistive effect (GMR) element such as a spin-valve MR element or with a tunnel magnetoresistive effect (TMR) element has been developed to more increase magnetic recording density, instead of a usual thin-film magnetic head with an anisotropic magnetoresistive effect (AMR) element.
At each test after manufacturing a thin-film magnetic head, it is necessary to confirm that the finished magnetic head with a MR element such as an AMR element, a GMR element and a TMR element may not produce any inherent noise such as Barkhausen noise.
Japanese Patent Publication 06150264 A1 of the same assignee as that of the present application discloses a MR head testing method. In this testing method, an external alternating magnetic field is applied, in a direction perpendicular to an Air Bearing Surface (ABS), to a plurality of MR thin-film magnetic heads which are aligned on a head block and are mechanically processed to control their gap-depth, and electromagnetic transfer characteristics (xcfx81-H characteristics) of each of the MR thin-film magnetic heads under the varying external magnetic field is measured.
U.S. Pat. No. 5,721,488 of the same assignee as that of the present application discloses an improved MR head testing method. In this latter testing method, alternating leakage magnetic field of a radio frequency, excited by a write current flowing through an inductive element is applied to the MR element in addition to external alternating magnetic field which is applied to the MR element in a direction perpendicular to the ABS of the head block, and xcfx81-H characteristics of each of the MR thin-film magnetic heads under the varying external and leakage magnetic fields is measured. By means of this testing method disclosed in the U.S. patent, since the applied leakage magnetic fields due to the write current will induce unstable state of the MR element, possible Barkhausen noise can be detected with a high probability.
However, in all conventional Barkhausen noise detection methods including the aforementioned testing methods, it is judged whether Barkhausen noise occurs or not by using measured result of xcfx81-H characteristics of the MR head element.
The measurement of xcfx81-H characteristics would be executed by using a low-frequency measurement device called as a xcfx81-H tester. In normal, whether Barkhausen noise occurs or not was judged by observing possible jump or hysteresis loop of the measured xcfx81-H shape.
However, according to these known testing methods of measuring xcfx81-H characteristics, it was very difficult to reliably detect whether Barkhausen noise is occurred or not because (1) the Barkhausen noise is not always visible, (2) a high speed noise may be temporarily produced but not continued, and (3) the shape of the noise is changed depending upon the intensity of the external magnetic field.
It may be effective to increase a possibility of occurrence of Barkhausen noise by forcefully applying before measurement to the head a strong magnetic field due to a large write current so as to change its basic state. However, impressing of a stress which will not ordinarily happen in the HDD to the magnetic head is undesirable. In order to surely detect the Barkhausen noise, it is desired that the measurement be executed under application of magnetic field as large as the produced magnetic field in the magnetic head. However, such application of the magnetic field is impossible during one time measurement, and also if sampling speed is low, it will fail to detect the Barkhausen noise.
It is therefore an object of the present invention to provide a method and apparatus for testing a thin-film magnetic head, whereby easy and reliable detection of Barkhausen noise in a short time can be realized.
Another object of the present invention is to provide a method and apparatus for testing a thin-film magnetic head, whereby the detection of Barkhausen noise can be attained without applying a stress to the thin-film magnetic head.
According to the present invention, a method for testing a thin-film magnetic head with a MR read head element includes a step of applying a low-frequency or DC external magnetic field to the thin-film magnetic head from a stationary magnetic field generation unit, a step of executing a high-frequency amplification of an output from the MR read head element under the application of the external magnetic field to provide a high-frequency amplified signal, a step of deriving only a high-frequency component from the high-frequency amplified signal to provide a high-frequency component signal, and a step of judging whether the thin-film magnetic head occurs a noise or not by using the high-frequency component signal.
Also, according to the present invention, an apparatus for testing a thin-film magnetic head with a MR read head element includes a stationary magnetic field generation unit for applying a low-frequency or DC external magnetic field to the thin-film magnetic head, a high-frequency amplification circuit for amplifying an output from the MR read head element under the application of the external magnetic field to provide a high-frequency amplified signal, a high-pass circuit for deriving only a high-frequency component from the high-frequency amplified signal to provide a high-frequency component signal, and a judgment unit for judging whether the thin-film magnetic head occurs a noise or not by using the high-frequency component signal.
A known low frequency (10 kHz or less) or DC external magnetic field of a known intensity is applied to the thin-film magnetic head from a stationary magnetic field generation unit. Under the application of the external magnetic field, an output from the MR read head element is amplified by a high-frequency amplification circuit, and then only a high-frequency component is derived from the high-frequency amplified output. In the derived high-frequency component, no low-frequency variable component corresponding to the applied external magnetic field is existed and Barkhausen noise if it is existed is appeared in a differentiated waveform. Thus, from this derived high-frequency component, whether a noise occurs or not can be easily judged in a short time. Further, since the output from the MR read head element is amplified by the high-frequency amplification circuit and the noise detection is performed using the high-frequency component derived from the high-frequency amplified output, it is possible to detect only high-speed changing characteristics of the MR element. In other words, it is possible to know high-speed behavior of the magnetic head, which could not be observed at all by using the xcfx81-H tester that is a low-frequency measurement equipment. Thus, detection of Barkhausen noise can be easily and reliably performed in a short time.
Furthermore, the detection of Barkhausen noise can be performed without applying magnetic stress to the thin-film magnetic head.
Since the external magnetic field applied to the magnetic head is that generated by the stationary magnetic field generation unit, no thermal noise and media noise is contained as the external magnetic field from the magnetic record media and therefore correct detection of Barkhausen noise can be expected.
Furthermore, since the external magnetic field intensity can be optionally adjusted by using the stationary magnetic field generation unit, it is possible to judge the safety when the magnetic head is assembled in the HDD, and also it is possible to obtain a jumped amount of Barkhausen noise. As a result, a ratio of the jumped amount with respect to the head output (S/N) can be obtained.
It is preferred that the judging step includes measuring the number of times that the high-frequency component signal exceeds a threshold, or that the judgment unit includes a comparison circuit for detecting that the high-frequency component signal exceeds a threshold to provide an output when detected, and a circuit for counting the number of the output provided from the comparison circuit. Since the output of the MR read head element is not sampled but amplified by the high-frequency amplification circuit and the derived high-frequency component is compared with the threshold, Barkhausen noise can be detected certainly without overlooking a momentary change of the output of the MR head element.
It is also preferred that the stationary magnetic field generation unit includes a unit for generating an alternating magnetic field with an intensity which corresponds to a level of an applied alternating current.
Preferably, an intensity of the external magnetic field applied from the stationary magnetic field generation unit is obtained at a time when it is judged that the thin-film magnetic head occurs a noise. For detecting Barkhausen noise, it is very important to know not only the existence of the noise but also a magnetic field intensity at which the Barkhausen noise will occur because of the aforementioned reason.
It is preferred that the method further includes a step of ignoring the judged occurrence of noise when the obtained intensity of the external magnetic field is out of a normal magnetic field range. Since the magnetic head cannot be specified that it will produce Barkhausen noise even under normal operation or under normal magnetic field, no counting of noise detection is executed at this time.
It is preferred that the method is performed for a plurality of thin-film magnetic head aligned on a head block.